1. Field of the Invention
The present invention relates to a hybrid working machine that uses both engine power and electric power.
2. Description of the Related Art
The related art will be described using an excavator by way of example.
As illustrated in FIG. 4, an excavator includes a crawler-type lower traveling structure 101 and an upper rotating structure 102 rotatably mounted, around a vertical shaft O, on the lower traveling structure 101. A working attachment A is mounted on the upper rotating structure 102.
The working attachment A includes a boom 103 capable of moving up and down, an arm 104 attached to the tip of the boom 103, a bucket 105 attached to the tip of the arm 104, and cylinders 106, 107, and 108 that are hydraulic actuators for driving the boom 103, the arm 104, and the bucket 105, respectively.
For a hydraulic excavator, a rotation hydraulic motor is used as a rotation actuator for rotating the upper rotating structure 102. In contrast, for a hybrid excavator, a rotation motor is used as the rotation actuator.
In a hybrid excavator, a hydraulic pump and a generator are driven by using an engine as a power source, and hydraulic actuators are driven by the hydraulic pump. At the same time, a rotation motor is driven by the generator and an electric storage device that is charged with power output from the generator (See Japanese Unexamined Patent Application Publication Nos. 2000-283107 and 10-42587).
In the hybrid excavator, the electric storage device may break down due to a failure in a relay, and may become unusable as a power source.
In such a case, there have been the following problems:    (i) Because the electric storage device, which is the power charging destination of the power output from the generator and which is one of the power sources of the rotation motor, becomes unusable, the voltage of a direct current (DC) bus connecting the generator (generator inverter) and the rotation motor (rotation motor generator) becomes unstable;    (ii) The power generation of the generator becomes insufficient for the power consumption of the rotation motor; and    (iii) As the engine load becomes greater, the output of the hydraulic pump becomes smaller.Accordingly, both the rotation and the hydraulic operation become unstable.